Process for cracking hydrocarbons containing hydrodesulfurized residual oil

ABSTRACT

In fluid catalytic riser cracking of a gas oil with a zeolite catalyst the inclusion of controlled amounts of residual oil in the feed results in an improvement in octane value and/or improvement in the distribution of octane value of the gasoline product. The octane number improvement is further enhanced by hydrodesulfurizing the residual oil prior to charging to the FCC unit. However, residual oil is relatively difficult to vaporize and the extent of its vaporization will depend on the equilibrium flash vaporization temperature at the bottom or inlet of the riser. Therefore, the quantity of residual oil in the feed must be controlled in relation to the equilibrium temperature in order to, on the one hand, vaporize and crack a sufficient quantity of residual oil to obtain the octane improvement effect, while, on the other hand, keeping the quantity of residual oil which remains unvaporized below a specified level.

United States Patent [1 1 Bryson et al.

[ PROCESS FOR CRACKING I-IYDROCARBONS CONTAINING I'IYDRODESULFURIZED RESIDUAL OIL [75] Inventors: Millard C. Bryson, Conway; Joel D.

McKinney, Indiana; Robert A. Titmus, Pittsburgh; Frederick K. White, Allison Park, all of Pa.

[73] Assignee: Gulf Research & Development Company, Pittsburgh, Pa.

[22] Filed: Jan. 10, 1972 [21] Appl. No.: 216,428

[52] US. Cl 208/120, 208/16, 208/61,

. 208/89, 208/113 [51] Int. Cl ..Cl0g11/04, ClOg 37/00, C101 H04 [58] Field of Search 208/120, 113

[56] References Cited UNITED STATES PATENTS 3,728,251 4/1973 Kelley et al 208/89 2,938,857 5/1960 Johnson et a1 3,055,822 9/1962 Honerkamp et al.. 3,671,420 6/1972 Wilson et a1 2,994,659 8/1961 Slyngstad et al 208/113 Dec. 25, 1973 Primary Examiner-Delbert E. Gantz Assistant Examiner-G. E. Schmitkons AttorneyMeyer Neishloss et al.

57 TRACT In fluid catalytic riser cracking of a gas oil with a zeolite catalyst the inclusion of controlled amounts of residual oil in the feed results in an improvement in octane value and/or improvement in the distribution of octane value of the gasoline product. The octane number improvement is further enhanced by hydrodesulfurizing the residual oil prior to charging to the FCC unit. However, residual oil is relatively difficult to vaporize and the extent of its vaporization will depend on the equilibrium flash vaporization temperature at the bottom or inlet of the riser. Therefore, the quantity of residual oil in the feed must be controlled in relation to the equilibrium temperature in order to, on the one hand, vaporize and crack a sufficient quantity of residual oil to obtain the octane improvement effect, while, on the other hand, keeping the quantity of residual oil which remains unvaporized below a specified level.

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The present invention applies to the type of riser cracking systems now in use employing riser outlet temperatures between 900 and 1,100F., a relatively This invention relate to the i oveme t i octane 5 low pressure which is not critical and can be about 5 to number of a gasoline product obtained by fluid zeolite catalytic cracking ofa feed without added hydrogen comprisinghydrodesulfurized residual oil in addition to gas oil.

Newermodel automobile engines are designed with lower compression ratios than heretofore in order to utilize no-lead or low-lead gasoline as fuel. These engines sometimes exhibit a hesitation upon acceleration from a full stop. This hesitation. arises due to uneven burning of fuel upon surge resulting in low fuel efficiency. This uneven burning is due to the presence of A the higher boiling gasoline components, i.e. gasoline components boiling above 360 or 380F. up to 400 or 430F., since these higher boilingcomponents include aromatic types which are difficult to ignite. The difficulty can be corrected by removal of these components. However, the high boiling gasoline components include some of the highest octane components in the full-range gasoline. The present invention tends to mollify the loss in octane value due to omission of 360 or 380F.+material from gasoline when a 360 or 380F. or lower end point gasoline is being produced by upgrading the octane value of the mid-boiling range fraction of the gasoline, i.e. that fraction of the gasoline boiling in the 125 to 300F. range, which includes the lowest octane fractionof the gasoline. This effect occurs independently of blending. When a full-range (100 or C to 400, 430 or 450F.) gasoline product is being produced the present invention can provide a generally higher octane value over the full-range gaso line product as well as contributing to more uniform octane values between the highest and lowest boiling fractions of the gasoline (the highest and lowest boiling fractions have a relatively high octane value) on the one hand and the middle boiling gasoline fraction (which has a relatively low octane value) on the other hand. Therefore, the present invention has utility in the production of both low end-point gasoline (360 or 380F.) or standard end point gasoline, i.e. 400F., or higher.

The presence of hydrodesulfurized (HDS) residual oil in the feed of the present invention provides animproved unleaded octane value in the fraction of the gasoline product boiling between 125 and 300F., generally, between 150 and 275F., more specifically, and between 175 and 250F., most specifically, as compared to the gasoline product from a feed comprising gas oil only and as compared to the gasoline productfrom a feed comprising gas oil plus the same amount of non-hydrodesulfurized residual oil. The improvement is achieved in unleaded and leaded Research and Motor octane values. It is an important advantage of this invention that the gasoline fractions improved in octane value are the fractions of the gasoline product possessing the lowest octane values. Inclusion of the residual or hydrodesulfurized residual feed can also increase the octane value of the lowest and highest boiling fraction of the gasoline product. Generally, however, the degree of improvement is greatest in the midboiling fraction. In any event, the full range gasoline 50 psig, a residence time of up to 5 seconds and catalyst to oil weight ratios between 4:1 and 2:1, generally, 5:] to 15:1 preferably, and 6:1 to 12:1, most preferably. Regenerated catalyst temperatures between 1,100F. and 1,350F., generally, and 1,225F. to 1,325F., preferably, can be employed. These riser systems operate with catalyst and hydrocarbon feed flowing concurrently into and through the riser at about the same velocity, thereby avoiding any significant slippage of catalyst relative to hydrocarbon in the riser and'avoiding formation of a catalyst bed in the reaction flow stream. In this manner the catalyst to oil ratio does not increase significantly from the riser inlet along the reaction flow stream. The catalyst employed is a fluid crystalline aluminosilicate molecular seive catalyst. Any catalyst of sufficient activity and/or selectivity to produce significant feed stock cracking at residence times of five seconds or less are within the purview of this invention.

Four tests were conducted to illustrate the effect of blending non-hydrodesulfurized residual oil into a gas oil distillate feed stream so that residual oil and gas oil were added together at the bottom of a riser. The tests were performed in a fluid catalytic riser cracking system employing a zeolite catalyst. The unit operating condition were held as constant as possible throughout all the tests. A uniform fresh feed rate was maintained during the tests. The reactor outlet temperature was maintained at 930F., the regenerator bed temperature at 1,220 to 1,235F. and the feed preheat temperature at 340 to 375F. Slurry recycle varied between 4.7 volume percent and 8.7 volume percent of fresh feed. The basic gas oil feed had a 5 percent distillation temperature of 577F. and a percent distillation temperature of 920F, and had a gravity of about 24API. The residual oil blended into the unit was percent non-hydrodesulfurized South Louisiana vacuum tower bottoms boiling above 1,050F. and having a gravity of l 1APl. As shown in Table 1, residual charge to the unit was absent in the base case and then was set at three incremental rates: 5.1 volume percent, 10.1 volume percent and 14.7 volume percent of the fresh feed. As the residual oil content was increased from the zero percent residual oil level to the 14.7 volume percent residual oil level, the hydrogen make increased from 0.32 volume percent to 0.73 volume percent and the coke make increased from 4.93 weight percent only to 5.47 weight percent, which is a very small increase. All of the light end components (methane through butane-butenes) decreased with increasing residual levels from a total volume percentage of 35.67 percent to 25.82 percent. Also, with increasing residual contents, the gasoline yield increased from 60.4 volume percent to 62.0 volume percent. The yield of products heavier than gasoline (light gas oil and decanted oil) showed an increase with increasing residual level, with the greatest portion of the change occurring with highest residual contents in the feed. Finally, with increasing residual feed content the conversion to gasoline and lighter products decreased from 80.2 volume percent to 75.3 volume percent. Selectivity to gasoline (measured as a ratio of TABLE 1 Gasoline octane ratings Gas oil plus Gas oil plus Gas oil plus Gas oil plus 0.0 percent 5.1 percent of 10.1 percent of 14.7 percent of Inspection residual 1050 F. residual 1050 F. residual 1050 F. residual Light gasoline Motor octane numbers Clear 80.4 80.0 80.4 80.9 +0.53 grams lead... 84.0 84.0 84.5 +1.59 grams lead... 87.0 85.9 84.3 85.5 +3.17 grams lead... 884 87.4 86.0 86.5 Research octane numbers Clear 92.0 92.4 91.8 92.5 +3.17 grams lead 99.6 92L- 100.1 100.5 Heavy gasoline Motor octane numbers I i V Clear 81.1 80.7 82.0 81.1 +0.53 grams lead... 83.6 84.6 84.0 84.5 +1.59 grams lead... 86.0 85.6 85.5 85.2 3.17 grams lead 87.7 87.2 87.4 86.8 Research octane numbers a Clear 93.0 92.8 92.7 95.1 98.6 98.0 98.7 98.5

+ 3.17 grams lead gasoline yield to conversion) increased from 0.753 for the gas oil feed to 0.823 for the highest residual-gas oil feed mix of the fourth test. Therefore, if proper conditions are achieved at the onset of the cracking reaction, i.e. at the bottom of the riser, the portion of the feed boiling higher than 1,050F. true boiling point is highly selective towards gasoline production. It yields increased gasoline selectivity in comparison to that yielded from the feed boiling below 1,050F., i.e. typical virgin gas oil distillate, at comparable cracking conditions.

As shown in Table 1, all the octane ratings remained essentially the same during the tests with only one exception. Table 1 shows that adding 5.1 percent or 10.1 percent of 1,050F.+ residual oil to the gas oil feed liquid did not affect the octane number of the gasoline product. An effect on octane number appeared for the first time wHehi41'7" percent of l,O50F.+ residual oil was added to the gas oil feed and then the effect was apparent only in the heavy gasoline product fraction (unleaded) and not in the light gasoline product fraction. Table 1 therefore indicates that as much as 10.1 percent residual oil had no effect upon the octane number of either light or heavy gasoline product and an effect did not appear until 14.7 percent of residual oil was added, whereupon the effect was apparent only in the heavy gasoline fraction and amounted to more than two Research octane numbers on an unleaded basis. In

' the data of Table l, the light gasoline fraction had a 5 percent temperature of 110F. and a 90 percent temgasoline product octane value of the amount of residual oil in a gas oil distillate feed stream. FIGS. 1 through 5 illustrate how to determine whether the amount of 1,050F.+ residual oil present in a feed stock to a riser operating at a particular feed-catalyst equilibrium temperature at the riser inlet will provide an octane number improvement in tests as illustrated in Table 1. In order for the residual oil to affect the product octane value, the high boiling residue feed components must vaporize at the riser inlet because only by vaporization can any feed component be cracked to gasoline and other products and exert an advantageous effect upon the process. Most residual feed components that do not vaporize remain on the hot catalyst surface and tend to be converted to coke, thereby resulting in a loss of useful product and a lowering of catalyst activity. This deleterious effect occurs in addition to the deposition upon the catalyst of the metals content of the residual oil, especially nickel and vanadium, further tending to reduce catalyst activity and selectivity.

vaporization of feed, especially of high boiling residual feed components, if it is to occur, must occur at the riser inlet where the catalyst is fresh from the regenerator and is therefore the hottest and most active and most selective. The riser temperature drops along the riser length due to heating and vaporization of the feed, the slightly endothermic nature of the cracking reac tion and heat loss to the atmosphere. Because nearly all the cracking in the system occurs within one or two seconds, it is necessary that feed vaporization occur nearly instantaneously upon contact of feed and regenerated catalyst at the bottom of the riser. Therefore, at the riser inlet, the hot, regenerated catalyst and preheated feed, generally together with a mixing agent such as steam, nitrogen, methane, ethane or other light gas, are intimately admixed to achieve an equilibrium temperature nearly instantaneously. This equilibrium temperature is referred to herein as the equilibrium flash vaporization temperature of the feed because it is in the process of achieving this temperature that all the feed components that are to vaporize do vaporize. At this equilibrium temperature substantially complete heat exchange between all materials fed to the riser is achieved. and all these materials achieve about the same temperature level, i.e. the hot, freshly regenerated catalyst is cooled to the equilibrium temperature by contributing the sensible heat required to raise the feed liquid to the temperature of vaporization, by contributing latent heat of vaporization and by contributing superheating heat to heat the vaporized, liquid above the temperature of vaporization. At the completion of this heat exchange process at the riser inlet, the catalyst, the mixing fluid, the vaporized feed and the unvaporized feed are all at about the same temperature, which is the equilibrium flash vaporization temperature of the system.

It is desirable for the equilibrium flash vaporization temperature to be as high as possible in order to vaporize as much of the feed as possible. It is best to achieve this high equilibrium temperature by utilizinga high regenerator temperature. However, the regenerator temperature is limited by many factors inherent in the system such as susceptibility of the catalyst to sintering and deactivation, equipment metallurgical temperature limitations, the amount of carbon on the deactivated catalyst, etc. A much less desirable method of increasing the equilibrium temperature is by arbitrary increase of catalyst feed rate because high catalyst-to-oil ratios are known to reduce selectivity to gasoline product by increasing production of undesirable products. Therefore, in any given reactor-regenerator system operating in a heat-balance relationship the equilibrium flash vaporization temperature should be held to the lowest practical level if an increase must be made at the price of increasing the catalyst-to-oil ratio.

Since the equilibrium flash vaporization temperature is essentially fixed by the regenerator temperature, the oil feed rate, the oil preheat temperature and the catalyst to oil ratio, in accordance with the present invention the quantity of residual oil which is vaporized at a given equilibrium temperature is established by adjusting the proportion of residual oil to gas oil boiling range material in the feed. The equilibrium flash vaporization temperature will be between the regenerated catalyst temperature and the riser outlet temperature. At the equilibrium flash vaporization temperature, essentially all the gas oil boiling range material having a true boiling point above the gasoline product and below 1,050F. will be vaporized and all or most of the 1,050F.+ residual oil will also be vaporized. It is only vaporized material that can be cracked to lighter useful products such as gasoline and furnace oil since, as stated, the unvaporized material tends to be converted to coke on the surface of the catalyst.

In accordance with this invention the amount of hydrodesulfurized residue having a true boiling point above l,050F. in the gas oil liquid feed is always only a minorproportion (much less than 50 volume percent, preferably less than 20 or 25 to 30 or 35 volume percent) of the hydrocarbon feed, and most preferably between and 21 percent, while the material having a true boiling point below l,050F. constitutes the remaining, major proportion of the liquid feed. The ratio of 1,050F.+ hydrodesulfurized residue to 400F. or 430F. to l,050F. gas oil feed (gas oil comprises the hydrocarbons boiling above gasoline but below the 1,050F.+ residue) is'adjusted in accordance with this invention either by blending a stream of 1,050F.+ hydrodesulfurized residual material into a gas oil distillate stream or by preparing the total feed directly by adjusting the distillation or flash temperature of a total crude or a reduced crude distillation unit preceding hydrodesulfurization. In either case, the proportion of hydrodesulfurized 1,050F.+ residue to lower boiling feed must be sufficiently low on the one hand that at least volume percent of the total feed is vaporized at the equilibrium flash vaporization temperature. Preferably, percent of the total feed is vaporized. Most preferably, at least 98 or 99+ percent of the total feed is vaporized. On the other hand, the ratio of 1,050F.+ hydrodesulfurized residue to lower boiling feed must be sufficiently high in relation to the equilibrium temperature that adequate l050F.+ hydrodesulfurized residue is vaporized and thereby available to be cracked to and improve the octane value of the gasoline product as compared to the octane value that can be achieved under the same conditions and at the same equilibrium temperature in the absence of the 1,050F.+ residue. Sufficient residue should be vaporized at least to improve the octane value of the heavy (250 to 400F.+ B.P.) gasoline fraction and preferably to also improve the octane value of the lighter (100 to 250F. B.P.) gasoline fraction. If 100 percent of the feed is vaporized at the equilibrium temperature the equilibrium temperature must be sufficiently high so that the proportion of l,050F.+ vapors in the total vapors is adequate to improve the octane number of the gasoline product.

The criticality to the present invention of slight variations in l,050F.+ residue content in the hydrocarbon feed is appreciated by referring to the accompanying figures. FIG. 1 shows the volume percent ofa particular total hydrocarbon feed vaporized at various temperatures. The hydrocarbon feed is the nonhydrodesulfurized bottoms of an atmosphere distillation tower employed as an FCC feed. The solid curve shown in FIG. 1 represents the true boiling point characteristics while the dashed curve represents the equilibrium flash vaporization characteristics of the feed. For the feed shown in FIG. 1, there is a substantial difference in the characteristics of the feed according to the two curves and FIG. 1 illustrates that for the purposes of the present invention it is the equilibrium flash vaporization characteristics that are controlling.

Assuming the equilibrium temperature at the bottom of the riser is fixed at l,050F.-by process conditions, then FIG. I shows that the feed at the bottom of the i r 91 99 ba r bas s wi l qqrnafisefiZ barm s t vaporized gas oil having a true boiling point below l050F., l3 barrels of vaporized 1,050F.+ true boiling point residue and 5 barrels of unvaporized residue having a true boiling point above l,050F. The data shown in Table 1, above, show that the octane improvement effect of the present invention in the particular tests made only begins to become manifest at a residual content in the feed between 10.1 percent and 14.7 percent, and, even then, the octane improvement effect appeared in the heavy gasoline fraction only. Therefore, at an equilibrium temperature of 1,050F. with the feed shown in FIG. 1, the 13 barrels of l,050F.+ residue which is vaporized would be on the borderline of effectiveness when charging the particular feed shown with the particular catalyst at the particular riser outlet temperature of the tests of Table 1.

FIG. 2 shows an FCC feed stock of only a slightly different composition as compared to the feed stock of F IG. 1. FIG. 2 represents the characteristics of the nonhydrodesulfurized bottoms of an atmospheric distillation or flashing of a crude oil. As shown in FIG. 2, at an equilibrium temperature of 1,050F., on a basis of 100 barrels of feed, 85 barrels of gas oil having a true boiling point below l,050F. were vaporized, l l barrels of l,050F.+ true boiling point residual oil were vaporized and 4 barrels of residual oil were notvaporized. Comparing FIGS. 1 and 2, it is seen that feed stocks providing a slightly different extent of residual vaporization at the equilibrium temperature can be prepared, one of which might provide the octane improvement of this invention while the other might not, based upon the conditions of the tests of Table 1.

In sharp contrast to the feed stocks of FIGS. 1 and 2, FIG. 3 shows the true boiling point data and flash equilibrium data of a non-hydrodesulfurized atmospheric tower bottoms FCC feed stock which is totally unsuitable for purposes of this invention. As shown in FIG. 3, at a 1,050F. equilibrium temperature, on a 100 barrel basis, the feed comprises 60 barrels of gas oil vapor having a true boiling point below 1,050F., only 7 barrels of residual oil vapor having a true boiling point above l,050F. and 33 barrels of unvaporized residue having a true boiling point above 1,050F. The feed stock of FIG. 3 is unsuitable for use at a 1,050F. equilibrium temperature because more than 10 percent is unvaporized. However, the very important comparison is made between the feed stock of FIG. 3 and the feed stocks of FIGS. 1 and 2, that there is considerably less 1,050F.+ residue vaporized at the same equilibrium temperature in the case of the feed stock of FIG. 3 than in the case of the feed stocks of FIGS. 1 and 2, even though the feed stock of FIG. 3 contains a considerably higher proportion of 1050F.+ residue than the feed stocks of FIGS. 1 and 2. FIG. 3 shows that when the quantity of 1,050F.+ residue in the feed exceeds 40 or 50 percent, the amount of 1,050F.+ residue that can be vaporized decreases. Therefore, the present invention does not apply to feeds having excessively high residue contents.

A comparison of FIG. 3 with FIGS. 1 and 2 therefore shows that the amount of 1,050F.+ material vaporized in the riser does not increase merely by increasing the absolute quantity of l,050F.+ true boiling point material in the feed. The comparison of the figures shows that the amount of 1,050F.+ true boiling point residue actually vaporized at a particular equilibrium temperature can actually decrease by increasing the proportion of said residue in the feed, and vice verse. This is a highly important observation in accordance with the present invention because it is the quantity of l,050F.+ residue actually vaporized which improves the octane value of the product and it is shown that the quantity of residue vaporized is not necessarily related to the proportion of residue in the feed at a fixed equilibrium temperature.

FIG. 4 is based upon the feed stock of FIG. 1 and FIG. 5 is based upon the feed stock of FIG. 3. FIGS. 4 and 5 relate the percentage of the feed vaporized to both equilibrium temperature and pressure in the riser. FIG. 4 indicates that with a regenerated catalyst temperature of l,l70F. it is possible to obtain a high de gree of vaporization with the particular feed tested. In contrast, FIG. 5 shows that with a generally similar regenerated catalyst temperature of l,l80F., it is not possible to obtain sufficient feed vaporization to render the particular stock useful as an FCC feed in the process of this invention.

Although the data illustrated in FIGS. 1 through 5 relate to non-hydrodesulfurized residual oils, similar results would appear if the residual oils were hydrodesulfurized.

It has heretofore been well known that hydrogeneration of a cracking feed increases conversion to cracked products. For example, aromatics are more refractory during cracking than the corresponding partially or completely saturated ring structures. On the basis of the prior knowledge, it would be expected that any hydrogenation of aromatics which lessened refractoriness would also reduce the octane value of the debutanized gasoline product since addition of hydrogen to aromatics lowers the octane value of the aromatics. It is the unusual feature of the present invention that hydrodesulfurization of a residual feed not only greatly increases the conversion to gasoline during a subsequent gasoline cracking step but also, unexpectedly, greatly enhances the octane value of the gasoline product. Although we are not bound by any theory, one explanation for the octane improvement effect is that during the hydrodesulfurization step or steps the splitting of carbon-sulfur bonds in the condensed aromatic ring structures of the high boiling propane-insoluble asphaltene and resin molecules results in a severing of lower boiling aromatics from the high boiling molecules so that there is an in situ net production of gas oil range aromatic hydrocarbons during the hydrodesulfurization process. Since aromatics are known to be high octane gasoline constituents, the gas oil derived from a residue via hydrodesulfurization is likely to be highly superior to a distillate gas oil as an FCC feed. The production of aromatics during residual hydrodesulfurization is shown in Ser. No. 205,791, filed Dec. 8, 1971, J. D. McKinney and J. A. Paraskos, entitled Production of Low Sulfur Asphaltic Fuel Oil, which is hereby incorporated by reference.

The hydrodesulfurization catalyst employed in the process of the present invention is conventional and comprises, for example, Group VI and Group VIII metals on a non-cracking support. Thus, the catalyst may comprise nickel-cobalt-molybdenum or cobaltmolybdenum on an alumina support. The alumina may be stabilized with a l to 5 percent by weight of silica. The preferred catalyst is a nickel-cobalt-molybdenum on alumina containing less than one percent by weight of silica which catalyst may or may not be sulfided. Magnesia is also a non-cracking support. An especially preferred catalyst comprises a particulate catalyst comprising particles between about one-twentieth and onefourtieth inch in diameter, such as described in U. S. Pat. No. 3,562,800 to Carlson et al., which patent is hereby incorporated by reference. The hydrodesulfurization can occur in either one stage or in two stages in series as described in the above-mentioned McKinney et al. application. The hydrodesulfurization process can reduce a hydrocarbon residual feed containing four or more percent by weight percent of sulfur to a product containing one percent, or even 0.5 or 0.3 weight percent sulfur.

The hydrodesulfurization pretreating step for the fluid catalytic cracking feed of this invention occurs with a hydrogen partial pressure of 1,000 to 5,000 pounds per square inch, preferably 1,000 to 3,000 pounds per square inch. A hydrogen partial pressure of 1,500 to 2,500 pounds per square inch is especially preferred. The gas circulation rate can be between about 2,000 and 20,000 standard cubic feet per barrel, generally, or preferably about 3,000 to 10,000 standard tageous effect of the hydrodesulfurization feed treatment of the present invention in preparing a residual oil for fluid catalytic cracking (FCC).

Data were obtained from tests with a 22 percent reduced Kuwait crude to illustrate that hydrodesulfurization pretreatment of a residual feed, which is initially unsuitable for use as an FCC charge stock for purposes of the present invention because the ratio of residual hydrocarbons to gas oil hydrocarbons therein is too high, can convert the residual oil into an FCC feed which has a greatly reduced ratio of high boiling residual to gas oil range hydrocarbons. In this manner, an FCC feed is prepared in which the gas oil hydrocarbons are totally or partially derived from the residual oil components in the original feed. If the gas oil hydrocarbons are partially derived from the residual oil components in the original feed, the remainder of the gas oil is either a distillate gas oil or is a portion of the bottoms from the distillation or flashing of the crude from which the residual oil is'derived, in which case it is hydrodesulfurized together with the residual oil. The tests were performed with a 22 percent reduced crude initially containing only 10 percent of material boiling below 1,000F. but as desulfurization proceeded through incremental stages the amount of material boiling below 1,000F. which was derived from the 1,000F.+ residue increased to 59 percent. The hydrodesulfurization was performed by the method of the aforementioned Mc- Kinney et al. application. The results of the tests are shown in the following table.

Percent Desulfurization Feedstock* 12 56 85 AW 6.5 8.7 17.7 20.3 Percent S 5.43 4.77 1.41 0.83 Percent N 0.39 0.39 0.20 0.20 Percent carbon residue 16.00 18.13 6.98 4.85 Percent asphaltenes 15.20 16.40 2.20 1.65 Ni (ppm) 33 36 3.7 0.4 V (ppm) 99 105 1.3 0.5 Percent boiling below 1000F. 10 26.5 44 59 Boiling range 556- 5 14-- 509-- 466- 1400F.+ 1400F.+ 1400F.+ 1400F.+

"22 percent reduced crude v The above data show that a residual feed stock initially grossly deficient in gas oil range hydrocarbons for use as an FCC feed stock in accordance with the present invention can be converted into a more useful FCC feed stock which is not only advantageous lower in sulfur, nickel and vanadium contaminants but also is con-' siderably richer in gas oil boiling range hydrocarbons and these gas oil boiling range hydrocarbons are advantageously derived from the original 1,000F.+ residue. The data indicate that considerable gas oil boiling range hydrocarbons can be generated from the original residue. Advantageously, production of gas oil hydrocarbons coincides with removal of contaminants. The amount of gas oil range hydrocarbons generated may be sufficient to convert the residue to a suitable FCC feed at the desired flash equilibrium temperature in the FCC riser. Otherwise, the hydrodesulfurization effluent can be converted to a suitable FCC feed stock upon blending with gas oil boiling range hydrocarbons from an extraneous source, such as a distillate stream.

A series of cracking tests were conducted in a. riser with a fluid zeolite catalyst to illustrate more fully the octane improvement advantage of the present inven tion. 'All of the tests were performed under the following general conditions:

Cracking temperature: F. 940-1025 Contact time: seconds 0.43.6 Reactor pressure: psig 23-30 Recycle rate: vol. percent of fresh feed 05.0 Catalyst-to-oil ratio: fresh feed 7-1'. Regenerator temperature: F. 1 140-1 160 Carbon on regenerated catalyst:

wt. percent 0 1 -035 Dispersion steam: lbll,000 lb catalyst 2-9 Stripping steam: lb/1,000 lb catalyst 4-8 Feed preheat temperature: F. 250650 The tests were performed with a South Louisiana full-range virgin gas oil, a South Louisiana nonhydrodesulfurized atmospheric tower bottoms and with the 650F.+ product of hydrodesulfurization of South Louisiana atmospheric tower bottoms. Following is a description of each of these charge stocks.

Hydrode- South Sulfurized Louisiana South South Full-range Louisiana Louisiana Virgin Atmospheric Atmospheric Gas Oil Tower Tower Bottoms Bottoms Gravity:

Sulfur:

wt. percent 0.58 0.39 0.05 Carbon residue:

wt. percent 0.19 3.06 1.40 Total nitrogen:

ppm 640 1,300 1.000 Aniline point: F. 186 192 204 Nickel: ppm 0.1 4.7 0.1 Vanadium: ppm 0.1 1.2 0.1 D1160 distillation: F. at

The following is a tabulation of yield based on feed obtained when each of the above charge stocks was cracked at 1,000F. to obtain percent conversion with a regenerated catalyst temperature of 1,140F.

Hydrodesulfurized Atmospheric Atmospheric Charge Stock: Gas Oil Tower Bottoms Tower Bottoms Sulfur, weight percent 0.58 0.39 0.05 Yields: vol. percent of fresh feed to FCC Debutanized gasoline 55.5 52.0 56.5 C 4.7 4.0 2.5 C,,+ gasoline 45.0 41.5 45.5 Total C 17.5 16.5 18.0 C 9.0 7.0 7.5 Total C, 11.0 10.5 12.0 CF- 8.(] 6.5 7.5 Total C liquid 109.0 104.0 1 11.5 C, and lighter: wt. percent 4.0 4.0 3.0 Coke: wt. percent 4.0 11.0 7.0 Selectivity to gasoline: Gasoline/conversion 0.74 0.69 0.75 Potential alkylate: vol. percent C: and C 29.7 23.6 26.3 C, through C, 39.4 31.8 31.4 Total gasoline: vol. percent C,-C. alkylation 85.2 75.6 82.8 C -C alkylation 90.2 79.8 85.4

The above table shows that the virgin atmospheric tower bottoms produced less debutanized (C gasoline and less total C liquid based on FCC feed than a gas oil distillate. This result is to be expected because In considering the data of the above table it should be understood that when the original crude oil is prepared for feeding to the hydrodesulfurization or FCC unit, a substantial yield of straight run naphtha and the ATE is a residue and therefore contains high boil- 5 lighter material is first flashed from the feed. Only that ing compounds (not present in a gas oil distillate) portion of the feed which boils above the gasoline which are difficult to vaporize and tend to coat and carrange must be cracked to the gasoline range in the FCC bonize a hot FCC catalyst. The residue also contains unit. The above table shows that a distillate gas oil FCC metals and sulfur which might further tend to deactifeed contains 32 volume percent of the crude and omits vate the catalyst. However, the above table surprisingly 0 the highest boiling 9 volume percent of the crude which shows that the hydrodesulfurized atmospheric tower is sent to a coker or a visbreaker. The atmospheric residue (HDS-ATB) when utilized as an FCC feed retower feed to FCC contains the heaviest 41 percent of sulted in a greater debutanized gasoline yield and a the crude while the hydrodesulfurized residue feed to greater total C liquid yield than even the gas oil, FCC contains the same portion of the crude diminished which is free of residue. This result is surprising beby about one volume percent to account for a 97.5 volcause it shows that it is possible to obtain more gasoline ume percent yield from the HDS unit. This 97.5 perrange FCC product based on FCC feed from a residue cent HDS yield is very high and is achieved due to the than from a distillate. The results are additionally sur- Occurrence of very little hydrocracking of carbon to prising because data presented above show that the h carbon bonds (as distinguished from carbon to sulfur drodesulfurization process does not essentially lower bonds) 1111116 HDS P The above table shows [hat the boiling range of the residue. The above table shows a higher debutanized gasoline C yi is a t, as expected, the FCC operation is ordinarily not geously achieved from the hydrodesulfurized residue improved by char in an di r r id t h FCC than from the non-hydrodesulfurized residue. Both the rise! but, unexpectedly, is improved by charging a residesulfurized and non-desulfurized residue feeds produe which is first hydrodesulfuri d, duced a higher FCC yield based on crude than the gas For the same tests, following is a tabulation of yield oil feed because the gas oil feed is a distillate and omits based upon the crude from whi h th FCC f d was d the highest boiling nine volume percent of the crude. rived. Again, for the same tests, the following table presents a tabulation of octane number for both the light and Basis of Comparison: 1000F. riser cracking to obtain heavy gasoline product fractions. Thee light gasoline V l5 p e rc ent conversion W product fraction has a boiling range of about 100 to Gasoline product Light-- Heavy Hyd a Hydrodesulfurized desulfurized Atmospheric atmospheric Atmospheric atmospheric Charge stock Gas oil tower bottoms tower bottoms Gas oil tower bottoms tower bottoms Micro octane number: Research, clear 90.8 92.5 93.1 93.2 94.4 93.9 Do +0.5g.... 94.7 96.8 96.3 95.3 96.7 95.9 Do +3.0g.... 99.6 101.2 100.2 98.0 99.1 99.1 Motor, clear 79.0 80.0 80.9 82.2 82.0 82.0 Do +0.5g.. 83.3 82.4 85.2 83.7 83.3 85.3 Do +3.0g 88.4 86.9 87.6 86.6 85.7 87.3 l eag usceptibilityz g r t 8.8 8.7 7.1 4.8 4.7 5.2 9.4 6.9 6.7 4.4 3.7 5.3

11.8 12.5 12.2 11.0 12.4 11.9 11.4 14.4 11.1 11.6 13.4 10.6 11.2 1413 12.6 11.4 13.4 11.8 Volume percent:

, Aromatics p 5 7 10 Q4 59 59 29 42 43 5 16 17- Saturates 66 51 47 31 4 H25, 24

g z 'g 250F. while the heavy gaspline fraction has a boiling Atmospheric Atmospheric range of about 250 to 430 F. Charge Stock: Gas Oil Tower Bottoms Tower Bottoms Th aEove d ta h h i accordan'c'e' with the Position on crude: vol. percent Yield on crude: vol. percent Gasoline yield from FCC:

vol. percent on crude Debutanized gasoline FCC gasoline alkylate:

C C alkylation 27.26 C -C, alkylation 28.86 "HDS yield of 97.5 volume percent method of the present invention an improvement in octane value is achieved in both the light and heavy gasoline product when cracking a residual oil-containing charge in accordance with the present invention as compared to a distillate oil charge which does not contain residual oil. The higher octane value for the atmospheric tower bottoms product is seen to be accounted for by either increased aromatics, increased olefins or a beneficial combination of the two.

The surprising feature of the data of the above table resides in the hydrodesulfurized atmospheric tower bottoms data which indicate generally higher octane and generally higher aromatics and/or olefins content as compared to non-hydrodesulfurized atmospheric tower bottoms. As indicated above, it is highly surprising that a hydrogenation pretreatment step, which would be expected to lower aromatics and olefin content, was found to accomplish the reverse when followed by a cracking step, i.e. increase or at least hold stable aromatics and olefins content so that a higher gasoline yield with a residual oil feed is accomplished together with a gain or at least no loss in octane value of the gasoline product.

Further tests were performed wherein the FCC feed was derived from the atmospheric tower bottoms (ATB) of a Kuwait crude rather than from a South Louisiana crude. Kuwait crudes are much richer in sulfur than domestic crudes. The additional tests are illustrated in Table 2. In one test of Table 2, a 1.0 weight percent sulfur feed from a single stage hydrodesulv FCC CHARGE QUALITY Produce From HDS of Kuwait 1.0% S Product From Charge Description: HDS of Kuwait ATB ATB Gravity: AP1 22.2 21.4 Sulfur: weight percent 0.5 1.0 Nitrogen: weight percent 0.17 0.18 Aniline Point, D611: F. 195.0 191.0 Nickel: ppm 4.4 9.6 Vanadium: ppm 7.0 14.0 Distillation, Vacuum, D1 160: F.

10% 723 681 864 788 50% 982 895 Cracked at 995 993 TABLE 2 Hydrodesulfurized Kuwait Atmospheric Tower Bottoms (0.5% (1.0% Charge Stock: Sulfur) Sulfur) Sulfur: weight percent 0.5 1.0 Regenerated catalyst inlet: F. 1120 1120 Riser outlet: "F. 1030 1030 Contact Time: seconds 0.6 0.6 Light gasoline RON-clear 91.3 90.0 +3 grams lead 99.8 98.9 MON-clear 78.1 78.3 +3 grams lead 85.8 85.6 Sulfur: wt. percent 0.006 0.01 Heavy gasoline RON-clear 89.7 893 +3 grams lead 97.8 95.7 MON-clear 79.9 78.5 +3 grams lead 86.4 83.1 Sulfur: wt. percent 0.07 0.19 Conversion 70.2 64.2 Yields: vol. percent of fresh feed/ Debutanized gasoline 56.1 48.2 Total C 15.3 14.2 C.= v 11.0 10.5 Total C;. 7.9 7.3 C,= 6.5 5.9 Total C 109.2 106.5 C and lighter: wt. percent 1.1 1.2 Coke: wt. percent 8.3 9.0 Selectivity to gasoline: Gasoline/conversion 0.8 0.75

Methane yield not included.

The data of Table 2 show that the additional degree of sulfur removal from the residue feed increased FCC debutanized gasoline yield from 48.2 to 56.1 percent and increased gasoline selectivity. Table 2 also shows that as the degree of desulfurization increased the octane number of the gasoline product also generally increased.

The advantage of the present invention is illustrated most strikingly in FIGS. 6 through 9. These figures contain graphs based on the same series of tests tabulated above showing the relationship of both Research and Motor Octane Numbers to the particular gasoline product fractions whose mid-points have the indicated true boiling points for both 950 and 1,000F. riser outlet cracking temperatures. The graphs of H68. 6 through 9 show that the gasoline product of gas oil cracking exhibits the highest octane value at its highest and lowest boiling extremity fractions but its mid-boiling fraction exhibits the lowest octane value, indicating a considerable imbalance in octane value in the gasoline product depending upon boiling point. The clip in octane value of the mid-boiling fraction can require that the midboiling fraction be further treated for upgrading, for instance, by constituting a reformer feed stock or by recycle to the FCC unit. Otherwise, as has been done commercially, the full range gasoline product is split into a high octane blending stock for premium gasoline pool blends and a lower octane blending stock for regular gasoline pool blends. FIGS. 6 through 9 show that the great advantage of the residual oil cracking process of the present invention is that the low octane value mid-boiling fraction is upgraded without diminishing the high octane values of the highand low-boiling extremity fractions, thereby imparting not only a higher average to the total gasoline product but also a better balance to the octane quality of the various incremental boiling fractions throughout the full boiling range of the gasoline product. This effect appears in FIGS. 6 through 9 as a tendency of the residual oil fraction to somewhat straighten the curve of octane value obtained from cracking a distillate gas oil at the same flash vaporization temperature and the straightening occurs by an improvement in'the octane value of the gasoline components having the lowest octane value. Therefore, a great advantage of the present invention will arise in the instance of the introduction of sufficient residual oil to a gas oil distillate feed to upgrade a mid-boiling fraction of the gasoline product to an extent that previous recycling or reforming of the midboiling fraction is no longer required, although such recycling or reforming can still be performed if further octane improvement is desired. Also, the need for gasoline splitting or fractionation could be eliminated, if desired, since the full range gasoline quality is adequate for use directly in premium gasoline blends. The residue (HDS) curves of FIGS. 6 through 9 show that a substantial further octane number improvement in residual cracking is achieved, in most cases, when the residual feed to FCC is first hydrodesulfurized. F108. 6 through 9 show that of all the feed stocks tested the hydrodesulfurized residual oil feed is the most capable of achieving the highly desirable result of a relatively even octane number quality over the entire boiling range of the gasoline product and is most capable of removing the octane number differential between the mid-boiling and highand low-boiling extremes of the full gasoline boiling range product.

FIGS. 6 through 9 show that the present invention is capable of producing a gasoline having improved uniformity of octane number along its boiling range comprising the cracked products of a single flash vaporization of a hydrodesulfurized crude oil bottoms comprising both gas oil boiling range hydrocarbons having a true boiling point below 1,050F. together with residual oil having a true boiling point above 1,050F. The cracked products of the gas oil boiling range hydrocarbons are present in the gasoline in greater proportion than the cracked products of the 1,050F.+ residual oil. A middle-boiling fraction of the cracked products in the gasoline derived exclusively from the gas oil boiling range hydrocarbons have a relatively low Research clear and/or Motor clear octane value as compared to higher and lower boiling fractions in the gasoline of the cracked products derived exclusively from the gas oil boiling range hydrocarbons. The proportion in the gasoline of the cracked products of the residual oil is sufficiently great to elevate by at least one number, or at least two or three numbers, on an unleaded basis, the Research and/or Motor octane value of the lowest 25, 50 or 100F. boiling fraction of the cracked products derived exclusively from the gas oil boiling range hydrocarbons. The gasoline of this invention can either be blended or unblended with other gasoline. It can be leaded but is advantageously unleaded or contains less than one gram of lead per gallon, although it can contain a greater quantity of lead.

EXAMPLE The data presented in Table 3 and illustrated in FIGS. 10 and 11 are based upon four FCC feed stocks prepared by atmospheric flashing of portions of a South Louisiana crude at 550, 650, 750 and 830F., respectively. The bottoms of each crude flash is designated as cut 1, 2, 3 and 4, respectively, and each cut is vaporized at four equilibrium temperatures; 1,000, l,025, l050 and 1,100F. The data of Table 3 show, in part I, the distillation data for having a true boiling point above the various riser equilibrium temperatures in both the feed liquid and the corresponding vapor for each cut.

The curves of FIGS. 10 and 11 are based upon the data of Table 3. These curves illustrate the high degree of interdependence between the concentration of residue in gas oil boiling range hydrocarbons in the feed liquid on the one hand and the equilibrium flash vaporization temperature at the bottom of an FCC riser on the other hand if an amount of residue is to be vaporized at the bottom of the FCC riser which is suflicient to enhance the octane value of the gasoline product. Assuming an FCC process wherein an improvement in gasoline octane value requires that more than 14 percent of the vapor at the bottom of the FCC riser have a true boiling point above the equilibrium flash vaporization temperature (as indicated by lines A in FIGS. 10 and 11), it is shown in FIG 10 that of the four feeds tested only one is capable of providing a vapor adequately rich in residue hydrocarbons while it is shown in FIG. 11 that of four flash equilibrium temperatures tested, only one is capable of providing a vapor adequately rich in residue hydrocarbons to achieve the octane improvement of the invention. FIG. 10 shows that cut 4, which is the FCC liquid feed which is richest in residue hydrocarbons of the four cuts tested, is not capable of producing a flashed vapor adequately rich in residue hydrocarbons. This is indicated by the fact that the curve representing cut 4 does not extend above line A of FIG. 10, although the curve of cut 3, which is poorer in residue, does extend above line A. FIG. 11 shows the high degree of interdependence between the residue concentration in the feed liquid and the riser equilibrium flash vaporization temperature because in the uppermost curve of FIG. 11, which is the only curve which extends above line A, only a feed containing between about 15 and 21 volume percent of residue boiling above the equilibrium temperature can produce a corresponding concentration of residue in the vapor which exceeds line A.

The cross hatched area B of FlGf l fl deflnes the interrn ediate concentration range of residue in the feed TABLE 3 i 1 Distillation,Data cm number: 1 2 3 4 550 F. Btms 650 F. Btms 750 F. Btms 830 F. Btms TBP EFV TBP EFV TBP EFV TBP EFV gDistillationz F. 1 s30 640 702 750 811 830 8 4 760 731 830 857 890 915 850 835 910 895 960 955 903 902 1000 960 1045 1010 1120 1016 1150 1045 1185 1087 1400 1090 II. FCC riser conditions (760 mm Hg) Feed cut number: 1 2 3 4 Riser q p: F. 1 100 1 9 s 0 10215 1000 i 1050 i025 1000 1100 1050 1025" 1100 10501025" 1000 Volume percent of liquid feed boiling above riser equil. temp:

Volume percent of vaporized feed boiling above riser equil. temp.:

liquid above and below which range the corresponding concentration of residue in the vapor is not sufficiently high to achieve the required octane improvement in the present example. FIG. Ml shows that the required corresponding concentration of residue in the vapor is not achieved unless the feed liquid contains a very narrowly circumscribed concentration range of said residue, i.e. a minimum of volume percent of said residue and a maximum of 21 volume percent of said residue.

A surprising observation regarding FlG. Ell is that as the equilibrium flash vaporization temperature increases, it is necessary to decrease the concentration of residuum in the feed and this combination of changes tends to increase the corresponding optimum percentages of the vapor having a true boiling point above the equilibrium temperature. Therefore, according to' the present invention, the required concentration of residue in the feed liquid trends downward as the equilibrium temperature increases. H6. 1 ll indicates that with the particular feed stocks shown the concentration of residue in the feed preferably should be below volume percent, i.e. the feed preferably should comprise at least 75 percent of gas oil. Therefore, FglG. it} and FIG. 11 both show it is possible to optimize residue content in the vapor by decreasing residue content in the feed liquid.

The advantage of an awareness of the method of this invention is further emphasized by comparing points C and D of FIG. 11. Although both points represent 14 percent of residue material in the vapor, point C accomplishes this residue vapor concentration with only 15 percent of residue material in the liquid feed while point D accomplishes this vapor concentration with 21 percent of residue in the feed. Since the additional'residue at point D will not be vaporized, it will tend to deposit on the hot catalyst and become coke. Therefore, the extra 6 percent of residue in the liquid feed at point D results in a higher coke make at point D than at point C. Where the FCC unit can achieve a heat balanced operation at a desired regenerated catalyst temperature, it is therefore apparent that the best mode of operating the present invention is to operate closer to point C than point D- We claim:

1. A process for fluidized zeolite riser cracking to a gasoline product at a residence time of less than five seconds of a hydrocarbon feed comprising a mixture of distillate gas oil boiling range hydrocarbons having a true boiling point below 1,050F. together with between 15 and volume percent of hydrodesulfurized residual oil hydrocarbons having a true boiling point above 1,050F., the hydrocarbon feed mixture and catalyst feed reaching an equilibrium flash vaporization temperature at the riser inlet, the proportion of the hydrodesulfurized residual oil in said feed mixture at said flash vaporization temperature both increasing the percent by volume of gasoline yield as compared to zeolite cracking of said distillate gas oil exclusively at said flash vaporization temperature and elevating by at least one octane number the 175 to 250F. product fraction as compared to zeolite cracking of said gas oil exclusively at said flash vaporization temperature.

2. The process of claim 3 wherein less than 109 percent of the feed hydrocarbons are vaporized at said equilibrium flash vaporization temperature.

3. The process of claim 3 wherein the gasoline product fraction having an enhanced octane number includes the lowest Research clear octane number fraction of the full-range gasoline.

4. The process of claim l wherein the gasoline product has a 380F. end point.

5. The process of claim 1 wherein the gasoline product has a 360F. end point.

6. The process of claim 1 wherein the gasoline product has an end point of 400F., or higher.

7. The process of claim l wherein the octane number is an unleaded Research or Motor octane number.

8. The process of claim 31 wherein the octane number is a leaded Research or Motor octane number.

9. The process of claim 1 wherein the hydrodesulfurized residue comprises 15 to 25 volume percent of the feed mixture.

10. A process for fluidized zeolite riser cracking to a gasoline product at a residence time of less than five seconds of a hydrocarbon feed comprising a mixture of distillate gas oil boiling range hydrocarbons having a true boiling point below 1,050F. together with hydpo 'asuruiiid residual oil hydrocarbons having a true boiling point above 1,050F., the hydrocarbon feed mixture and the catalyst feed reaching an equilibrium flash vaporization temperature at the riser inlet, controlling the concentration of 1,050F.+ hydrodesulfurized residue in the feed liquid mixture between 15 and 30 volume percent of the mixture in relation to said equilibrium temperature to obtain substantially an optimum concentration of said residue in the vapor at said equilibrium temperature, the proportion of the hydrodesulfurized residual oil in said feed mixture both increasing the percent by volume of gasoline yield as compared to zeolite cracking of said distillate gas oil exclusively at said flash equilibrium temperature and elevating by at least one octane number the 175 to 250 F. product fraction as compared to zeolite cracking of said gas oil exclusively at said flash equilibrium temperature.

11. The process of claim Ml wherein the gasoline product having the enhanced octane number includes the lowest Research clear octane number fraction.

12. The process of claim it) wherein the gasoline product has a 380F. end point.

13. The process of claim wherein the gasoline product has a 360F. end point.

14,. The process of claim 10 wherein the gasoline product has an end point of 400F., or higher.

15. The process of claim 10 wherein the reactor outlet temperature is 900 to 1,100F.

16. The process of claim Ml wherein the octane number is an unleaded Research or Motor octane number.

17. The process of claim Ml wherein the octane number is a leaded Research or Motor octane number.

18. The process of claim 10) wherein the hydrodesulfurized residue comprises 15 to 25 volume percent of the feed mixture.

T9. The process of claim Ml wherein the residue com prises 15 to 21 volume percent of the feed mixture.

is m a t a 

2. The process of claim 1 wherein less than 100 percent of the feed hydrocarbons are vaporized at said equilibrium flash vaporization temperature.
 3. The process of claim 1 wherein the gasoline product fraction having an enhanced octane number includes the lowest Research clear octane number fraction of the full-range gasoline.
 4. The process of claim 1 wherein the gasoline product has a 380*F. end point.
 5. The process of claim 1 wherein the gasoline product has a 360*F. end point.
 6. The process of claim 1 wherein the gasoline product has an end point of 400*F., or higher.
 7. The process of claim 1 wherein the octane number is an unleaded Research or Motor octane number.
 8. The process of claim 1 wherein the octane number is a leaded Research or Motor octane number.
 9. The process of claim 1 wherein the hydrodesulfurized residue comprises 15 to 25 volume percent of the feed mixture.
 10. A process for fluidized zeolite riser cracking to a gasoline product at a residence time of less than five seconds of a hydrocarbon feed comprising a mixture of distillate gas oil boiling range hydrocarbons having a true boiling point below 1, 050*F. together with hydrodesulfurized residual oil hydrocarbons having a true boiling point above 1,050*F., the hydrocarbon feed mixture and the catalyst feed reaching an equilibrium flash vaporization temperature at the riser inlet, controlling the concentration of 1,050*F.+ hydrodesulfurized residue in the feed liquid mixture between 15 and 30 volume percent of the mixture in relation to said equilibrium temperature to obtain substantially an optimum concentration of said residue in the vapor at said equilibrium temperature, the proportion of the hydrodesulfurized residual oil in said feed mixture both increasing the percent by volume of gasoline yield as compared to zeolite cracking of said distillate gas oil exclusively at said flash equilibrium temperature and elevating by at least one octane number the 175* to 250*F. product fraction as compared to zeolite cracking of said gas oil exclusively at said flash equilibrium temperature.
 11. The process of claim 10 wherein the gasoline product having the enhanced octane number includes the lowest Research clear octane number fraction.
 12. The process of claim 10 wherein the gasoline product has a 380*F. end point.
 13. The process of claim 10 wherein the gasoline product has a 360*F. end point.
 14. The process of claim 10 wherein the gasoline product has an end point of 400*F., or higher.
 15. The process of claim 10 wherein the reactor outlet temperature is 900* to 1,100*F.
 16. The process of claim 10 wherein the octane number is an unleaded Research or Motor octane number.
 17. The process of claim 10 wherein the octane number is a leaded Research or Motor octane number.
 18. The process of claim 10 wherein the hydrodesulfurized residue comprises 15 to 25 volume percent of the feed mixture.
 19. The process of claim 10 wherein the residue comprises 15 to 21 volume percent of the feed mixture. 